1. Field of the Invention
The invention relates to electrically connecting a plurality of respective closely spaced electrodes of two electronic devices and, more particularly, to electrically connecting the electrodes of light-emitting diode (LED) array chips to corresponding electrodes of an electronic control circuit.
2. Description Relative to the Prior Art
An LED array chip of the type used in an optical printhead comprises of row of uniformly spaced LED sites that can be individually energized to expose a photoreceptor or other image-receiving medium to reproduce an image pattern. The individual light sources are very small and closely spaced, e.g., 160 or more per centimeter, and a bonding pad of an energizing electrode for each LED source must be connected to a bonding pad of a corresponding electrode of a driver circuit. Typically, two driver chips straddle an array chip and electrodes from the LED sites extend alternately toward opposite edges of the LED chip to allow bonding to corresponding electrodes of the adjacent driver chip. Accordingly, if the LED array chip has 160 LED sites per centimeter, the bonding pads of 80 electrodes extend along each edge of the LED array chip and must be bonded individually to corresponding bonding pads of the adjacent driver circuit. The center-to-center distance of the electrode bonding pads is, therefore, on the order of 0.125 millimeters. Conventional wire bonding techniques, which are well known in the art, can be used to connect bonding pads with this spacing, but such spacing is close to the practical limits of this technique. U.S. Pat. No. 4,524,372, issued on June 18, 1985, shows the concept of staggering bonding pads or a conductor array to provide slightly more room for bonding each wire, but this bonding pad arrangement is not employed on the diode array chip itself, and there is no suggestion of employing such a staggered pad arrangement to allow the use of two different superimposed layers of connections. As disclosed in commonly assigned U.S. patent application Ser. No. 228,641, entitled "LED ARRAY PRINTHEAD WITH TAB BONDED WIRING", filed on Aug. 5, 1988, the use of tape-automated bonding (TAB) bonding techniques improves the ability to connect such closely spaced electrode bonding pads and avoids the potential short circuiting problem, but these techniques also become impractical when the bonding pad pitch becomes much smaller than about 0.075 millimeter. Accordingly, because it is now possible to produce LED array chips with diode sites significantly in excess of 160 per centimeter, the limit to the resolution of the array or the printhead is dictated by limitations on the ability to bond the LED electrodes to the corresponding electrodes of the driver chips.